Single wire serial interface

ABSTRACT

A single wire serial interface for power ICs and other devices is provided. To use the interface, a device is configured to include an EN/SET input pin. A counter within the device counts clock pulses sent to the EN/SET input pin. The output of the counter is passed to a ROM or other decoder circuit. The ROM selects an operational state for the device that corresponds to the value of the counter. In this way, control states may be selected for the device by sending corresponding clock pulses to the EN/SET pin. Holding the EN/SET pin high causes the device to maintain its operational state. Holding the EN/SET pin low for a predetermined timeout period resets the counter and causes the device to adopt a predetermined configuration (such as off) until new clock pulses are received at the EN/SET pin.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to control interfaces forintegrated circuits and other devices. More particularly, the presentinvention includes a single wire serial interface that may be used tocontrol power ICs and other devices.

BACKGROUND OF THE INVENTION

In power IC applications, an interface generally serves to managefunctions such as power level, or on and off switching. In the loadswitch power IC case, the IC either delivers power to a subsystem or notdepending on the state of the on/off pin. In a more complex power supplycontroller, the regulated output voltage is set by a more complexinterface such as an integrated 5-pin digital to analog interface. Whenmany subsystems exist within the same system, an even more complexinterface, such as the SMBUS interface may be implemented.

The complex power IC can easily afford a multi-pin control interface,since it is already in a large package, and has sufficient functionaldensity. The stand-alone power management function cannot normally offera complex control interface due to die size or package size constraints.Still there are cases where this type of control is desirable. Forinstance, it may be desirable to vary a current limit over differentload scenarios. However, few pins are available for control of thesimple load switch because most of the pins are used by the powerfunction, and there is no board space or budget for a larger package.Some functionality can be added by means of an analog interface, butsince most applications are controlled by a microprocessor, a digitalinterface is easiest to implement and most cost effective. A serialinterface is efficient, but common simple serial interfaces such as3-wire or 2-wire require too many pins. Complex serial interfaces suchas SMBUS are generally too complex and expensive to merit implementationfor the stand-alone power management function.

For these reasons and others, there is a need for an interface that maybe used to control stand-alone power and other IC types. Ideally, thisinterface would be able to accommodate a wide variety of control needsand be scaleable to many levels of complexity. Minimal pin use is alsodesirable, with the ideal being use of a single pin that may optionallybe shared with another function.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a single wire serialinterface that may be used to control stand-alone power ICs and otherdevices. For this aspect, an IC is configured to include a sensingcircuit, a counter, and a ROM or similar decoder. The sensing circuitmonitors the voltage present at one of the IC pins. Typically, this willbe the on/off pin and is referred to as the EN/SET pin. The sensingcircuit determines whether or not the voltage at the EN/SET pin is high,low, or toggling.

When the voltage at the EN/SET pin is toggling the counter is enabled.This causes the counter to count the rising edge of each clock pulsesent to the EN/SET pin. Holding the voltage at the EN/SET pin highcauses the counter to stop counting and maintain its value. Holding thevoltage at the EN/SET pin low for more than a preset timeout periodcauses the counter to reset to zero.

The ROM contains a total of 2.sup.n words of m bits. Each m-bit wordcorresponds to one control state for the IC. The output of the counteris an address within the ROM selecting a particular m-bit word andcontrol state. For simple functions, the counter can be only a few bits,in which case the counter outputs can be directly decoded in logicwithout the complexity of a ROM.

Another aspect of the present invention is an LED current source ICincorporating the single wire serial interface. The LED current sourceincludes at least one current output and one EN/SET input. For arepresentative implementation, the ROM includes a total of thirty-two(32) words. Each word corresponds to an output level for the one or morecurrent outputs. The output levels are preferably configured as alogarithmic scale, yielding two decades of output levels and LEDluminosity.

Another aspect of the present invention is a load switch ICincorporating the single wire serial interface. The load switch includesone EN/SET input and n outputs where n is greater than one. For the caseof the load switch, the bits in the counter may be used to directlycontrol the state of the individual outputs (i.e., each bit determinesthe state of a corresponding output). This allows the ROM to be omittedfrom the load switch IC, simplifying its design. The bits in the counteryield a total of 2.sup.n different output configurations (i.e., allpossible configurations).

Another aspect of the present invention is a current limited load switchIC incorporating the single wire serial interface. The current limitedload switch includes one or more outputs and one EN/SET input. Each wordin the ROM corresponds to a different current limit for the outputs.

Other aspects and advantages of the present invention will becomeapparent from the following descriptions and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and forfurther features and advantages, reference is now made to the followingdescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a timing diagram illustrating the use of the single wireprotocol according to one aspect of the present invention.

FIG. 2 is a block diagram showing an IC using a single wire serialinterface according to one aspect of the present invention.

FIG. 3 is a timing diagram illustrating the use of the single wireserial interface of the IC of FIG. 2.

FIG. 4 is a diagram showing a sensing circuit appropriate for use in theIC of FIG. 2.

FIG. 5 is a block diagram showing an IC using a latched implementationof single wire serial interface according to one aspect of the presentinvention.

FIG. 6 is a timing diagram illustrating the use of the single wireserial interface of the IC of FIG. 5.

FIG. 7 is a diagram showing a latch driver circuit appropriate for usein the IC of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention and their advantagesare best understood by referring to FIGS. 1 through 7 of the drawings.Like numerals are used for like and corresponding parts of the variousdrawings.

Single Wire Serial Protocol

An aspect of the present invention provides a single wire serialprotocol that may be used to control ICs and other compatible devices.To use the single wire serial protocol, a device must support a seriesof different operational states or modes. For one example, a stand-alonepower IC might be configured to support a range of different outputlevels. Typically, these output levels would progress in even incrementsfrom a no-power or off condition to a full power condition. Eachdifferent output level would define a particular operational state. Thesingle wire serial protocol allows the operational states of compatibledevices to be dynamically controlled. Thus, for the stand-alone power ICexample, the single wire serial protocol would be used to selectdifferent operational states and associated output power levels.

Devices that support the single wire serial protocol are configured toreceive an EN/SET signal. As shown by the timing diagram of FIG. 1, theEN/SET signal may be characterized as having three different waveforms.The first of these is a toggling waveform where the EN/SET signal iscomposed of a series of clock pulses. The second waveform is where theEN/SET signal is asserted to have a constant high value. The thirdwaveform is where the EN/SET signal is asserted to have a constant lowvalue.

The toggling waveform causes compatible devices to select particularoperational states. The total number of clock pulses (or rising edges)determines the particular operational state that will be selected (i.e.,four clock pulses selects the fourth operational state and so on).Additional clock pulses that exceed the number of operational statessupported by a compatible device will generally cause the count torollover and start again with the first operational state.

The constant high waveform causes compatible devices to maintain theirpreviously selected operational states. As shown in FIG. 1, the currentoperational state may be continued for an arbitrary duration in thisway.

The constant low waveform causes compatible devices to power off (orotherwise adopt a predefined configuration) after a pre-defined timeoutperiod has elapsed. The timeout period allows compatible devices todistinguish between the constant low waveform and the shorter lowportions of the toggling waveform. For a typical implementation, thetimeout value is 400 μs with the EN/SET signal having a frequency in therange of 1 Mhz to 10 kHz. Higher and lower frequencies are alsopossible.

Single Wire Serial Interface

To use the single wire serial protocol, compatible devices must providea single wire serial interface. For the purposes of illustration, FIG. 2shows a block diagram of an IC (generally designated 200) configured toprovide this interface. IC 200 includes one or more inputs 202 and oneor more outputs 204. IC 200 also includes an EN/SET input 206 and a coreportion 208. Core portion 208 is intended to be generally representativeof the circuits that function to create outputs 204 using inputs 202EN/SET input 206 is connected to a sensing circuit 210. Sensing circuit210 monitors the EN/SET signal at EN/SET input 206 and determines ifthat voltage is constantly high, constantly low, or toggling. Based onthis determination, sensing circuit 210 produces two signals: a Clocksignal and an Enable signal. The Clock and Enable signals control theoperation of a counter 212 having n bits. Counter 212 counts the risingtransitions of the Clock signal whenever sensing circuit 210 asserts theEnable signal. Counter 212 resets whenever the Enable signal is notasserted.

The relationship between the EN/SET signal and the Clock and Enablesignals is shown in more detail in the timing diagram of FIG. 3. Asshown in that figure, a rising transition of the EN/SET signal causessensing circuit 210 to assert the Enable signal. Sensing circuit 210holds the Enable signal high until the EN/SET signal transitions to alogical low state and remains in the low state until the predeterminedtimeout period has elapsed. The Enable signal acts to gate the Clocksignal. As long as the Enable signal remains high, sensing circuit 210forwards the EN/SET signal as the Clock signal. Counter 212 receivesboth the Clock and Enable signal. The first rising transition of theEN/SET signal raises the Enable signal and causes the EN/SET signal tobe forwarded as the Clock signal. Counter 212 responds by increasing itsvalue to one. Subsequent rising transitions causes Counter 212 toincrement its value to two, three and so on. Counter 212 resets to zerowhen sensing circuit 210 transitions the Enable signal to a low value.

The n output bits of counter 212 control a ROM 214. ROM 214 has a totalof 2.sup.n words, each having m bits. Each m-bit word corresponds to onecontrol state for IC 200. The n-bit output of counter 212 selects aparticular m-bit word within ROM 214. The selected control state andEnable signal are passed to core portion 208. Core portion 208 isconfigured to adjust its operation to match the selected control state.

Sensing Circuit

FIG. 4 shows a representative implementation for sensing circuit 210. Asshown in that figure, sensing circuit 210 produces the Enable and Clocksignals by timing the logic low period of the EN/SET signal. As long asthe timeout period is not exceeded, the Enable signal will remain high,and the EN/SET signal will, feed through logic gate AND1 to become theClock signal. In the described implementation, the timer consists ofcapacitor C1 and current source I1. Transistors MN2 and MN3 mirrorcurrent source I1. This linearly discharges capacitor C1 when the EN/SETsignal is a logical low, and transistor MP1 is off. If the EN/SET signalremains in a logic low state long enough, capacitor C1 will discharge toa voltage that is less than the threshold of transistor MN1 and turn MN1off. When MN1 is off, R1 pulls node “2” to the threshold of Schmittrigger ST1 and the Enable signal goes to a logic low state. As long asthe EN/SET signal remains low for a period less than the timeout period,the Enable signal will remain in a logic high state. The timeout periodis dominated by the power supply voltage, the threshold of transistorMN1 (V_(tMN1)), the value of capacitor C1, and the magnitude of currentsource I1, given by:

Timeout=C*(Vcc−V _(tMN1))/I1

Typical values of C1=10 pF, Vcc=5 v, V_(tMN1)=1 v and I1=0.1 μA yield atimeout period of 400 μs. Sensing circuit 210 can respond to a 400 nssignal of the EN/SET signal. As a result, it is able to differentiatebetween the EN/SET signal as Clock and EN/SET signal as Enable. Atypical application can be designed around a range of EN/SET frequenciesbetween 1 Mhz to 10 kHz, or slower if desired.

Latched Single Wire Serial Interface

Devices that implement the just described single wire serial interfaceselect a new control state each time a rising edge of a clock pulse isreceived. One result is that compatible devices progressively selecteach control state in sequence until the desired control state isreached. So, selecting the eighth control state means that compatibledevices will progressively select control states one through sevenbefore finally selecting the eighth (desired) control state. For somedevices this behavior is acceptable or even desirable. This can be true,for example where the device is a current source where progressivelyincreasing output can be benign or even useful. In other cases,selection of intermediate control states may have unwanted side effects.This could be true for the case of the multiple load switch that isdescribed below.

FIG. 5 shows a block diagram of an IC (generally designated 500) thatuses an implementation of the single wire serial interface thateliminates intermediate control states. IC 500 includes the majority ofcomponents previously described for FIG. 2 and IC 200. In this case, theoutput of counter 212 is passed through a latch 502 before reaching ROM214. Latch 502 is controlled by a Latch signal generated by a latchdriver circuit 504.

The relationship between the EN/SET, Clock, Enable and Latch signals isshown in FIG. 6. As shown, the Latch signal remains low until the EN/SETsignal has been maintained in a high state for a duration that exceeds apredetermined latch timeout period. Holding the EN/SET signal high forlonger than the latch timeout period causes latch driver 504 to assertthe Latch signal. This, in turn causes latch 502 to forward theaccumulated value of counter 212 to ROM 214. The result is that counter212 is prevented from forwarding intermediate control states until theEN/SET signal has been asserted high after the train of clock pulses hasbeen completed.

Latch Driver Circuit

FIG. 7 shows a representative implementation for latch driver 504. Asshown in that figure, latch driver 504 produces the Latch signal bytiming the logic high period of the EN/SET signal. As long as the EN/SETsignal is high for less than the latch timeout period, the Latch signalremains low. In other described implementation, the timer consists ofcapacitor C1 and current source I1. Transistors MN2 and MN3 mirrorcurrent source I1. This linearly discharges capacitor C1 when the EN/SETsignal is a logical high, and transistor MP1 is off. If the EN/SETsignal remains in a logic high state long enough, capacitor C1 willdischarge to a voltage that is less than the threshold of transistor MN1and turn MN1 off. When MN1 is off, R1 pulls node “2” to the threshold ofSchmit trigger ST1 and the Latch signal goes to a logic high state. Aslong as the EN/SET signal remains high for a period less than the latchtimeout period, the Latch signal will remain in a logic low state. Thelatch timeout period is dominated by the power supply voltage, thethreshold of transistor MN1 (V.sub.tMN1), the value of capacitor C1, andthe magnitude of current source I1, given by:

Latch Timeout=C*(Vcc−V _(tMN1))/I1

Typical values of C1=10 pF, Vcc=5 v, V_(tMN1)=1 v and I1=0.1 μA yield alatch timeout period of 400 μs. Latch driver 504 can respond to a 400 nssignal of the EN/SET signal. As a result, it is able to differentiatebetween the EN/SET signal as Clock and EN/SET signal as Latch. A typicalapplication can be designed around a range of EN/SET frequencies between1 Mhz to 10 kHz, or slower if desired.

Decoder

ROM 214 provides a mapping between the EN/SET signal and associatedcontrol states for IC 200. In some cases, there may be relatively fewcontrol states. In other cases, the mapping may be defined functionally.In these cases, it is possible to replace ROM 214 with a decoder. Thisallows the outputs of counter 212 to be directly decoded in logicwithout the complexity of a ROM.

LED Driver

The white LED has become the backlight source of choice for smalldisplays used in products such as cell phones that typically use alithium ion battery for power. The white LED is an excellent lightsource. However, it requires from 3.6 to 4.1 volts of forward biasvoltage to conduct current and emit light. Since the lithium ion batteryruns between 4.1 and 2.9 volts, a regulated boosted voltage must begenerated to power the LED. Four LED's are typically used in a display;either in a serial or a parallel arrangement.

The lowest cost solution is to drive the four LED's in parallel with acharge pump. The higher cost solution is to drive the four LED's inseries with a DC/DC boost converter capable of boosting the lithium ionbattery up to four times the forward voltage of the LEDs (e.g.4.times.4.1=16.4 volts). The DC/DC boost converter is higher cost due tothe cost and size of the required inductor, but since the LED is reallya current mode device, the performance is better because all of theLED's in series will be biased with the same current and share the sameluminosity.

The charge pump solution is attractive because small low cost capacitorscan be used to develop a voltage of up to 1.5 or 2 times the batteryvoltage. The disadvantage to the charge pump solution is that theresulting voltage must be sensed as a current for brightness control ofthe LED. A single voltage can drive multiple LED's, however only one LEDis used as the current reference. This is achieved by adding a currentsetting and sensing resistor in series with it. The additional LED'shave a matching resistor in series, but unless their forward voltagesmatch that of the reference LED, they will have substantially differentcurrents and, as such, brightness levels. A better solution would haveparallel current outputs for driving the LED with a current. In thismanner, all LED's would have the same bias current and luminosity. Theparallel outputs however, require more pins and a larger package that isa significant disadvantage.

Another issue is brightness control. Brightness control can be performedby setting a reference current and leaving it constant, or by applyingsome control means to the DC/DC converter to obtain a different outputvoltage or current. One way to control the brightness of an LED is tosimply turn it on and off at a higher frequency than the human eye candetect, and pulse width modulate (PWM) the on-time. An easier systemsolution would be an interface whereby a current control is input to theDC/DC converter to control the output current. This can be accomplishedeither by a control voltage or a digital interface. A simple solution isa digital interface, but to have enough resolution, or a large enoughrange, many bits of control are required. This leads again to higherundesirable pin count.

Since the human eye senses brightness logarithmically, a useful digitalcontrol would result in a logarithmic brightness scale. A logarithmicscale that adequately covers two decades of luminosity requires at least5-bits or 32 levels.

An aspect of the present invention provides an LED driver thateffectively meets all of these requirements. The LED driver ispreferably configured as a 12-pin device with four LED current sourceoutputs. The LED driver also includes an EN/SET input that supports thesingle wire serial protocol described above. The EN/SET input functionsas the on/off control as well as the brightness control. Internally, theLED driver includes a five-bit counter and a thirty-two word ROM. Thecontrol states included in the ROM are configured to providelogarithmically increasing levels of luminosity. The counter and ROM arescaleable to any number of levels beyond or below 32.

Multiple Load Switch

Another aspect of the present invention is a load switch ICincorporating the interface described in the preceding paragraphs. Foran eight-pin package, the load switch includes one EN/SET input, fiveoutputs, a power input and a ground input. For the case of the loadswitch, the bits in the counter may be used to directly control thestate of the individual outputs (i.e., each bit determines the state ofa corresponding output). This allows the ROM to be omitted from the loadswitch IC, simplifying its design. The bits in the counter yield a totalof 2.sup.5 or thirty-two different output configurations (i.e., allpossible configurations). If the load switches are very slow to respond,the single wire serial interface can be operated at a much higherfrequency than the switches can respond and the outputs will be wellbehaved. In the case where the switches are fast, an addition must bemade whereby the value clocked into the single wire serial interface isnot latched until the clocking has stopped.

Current Limited Load Switch with Configurable Current Limit

Another aspect of the present invention is a current limited load switchIC incorporating the interface described in the preceding paragraphs.The current limited load switch includes one or More outputs and oneEN/SET input. Each word in the ROM corresponds to a different currentlimit for the one or more outputs. The current limited load switch isdisabled a predetermined period after the EN/SET transitions to the lowstate.

Although particular embodiments of the present invention have been shownand described, it will be apparent to those skilled in the art thatchanges and modifications may be made without departing from the presentinvention in its broader aspects, and therefore, the appended claims areto encompass within their scope all such changes and modifications thatfall within the true scope of the present invention.

What is claimed is:
 1. An interface for controlling a device, theinterface comprising: a first circuit for accumulating a count of clockpulses encoded in a received signal; a second circuit for mapping thecount of clock pulses into a corresponding control state for the device;and a third circuit for resetting the count of clock pulses to zero ifthe received signal is low for a period that exceeds a predeterminedtimeout value.